1. Field of the Invention
The invention disclosed herein relates to an image forming apparatus, and particularly to a structure for correcting a displacement of a sheet in the width direction perpendicular to the sheet conveying direction.
2. Description of the Related Art
Conventionally, an image forming apparatus such as a copying machine, a printer and a facsimile forms a toner image on an image bearing member such as a photosensitive drum and an intermediate transfer belt, and transfers this toner image onto a sheet conveyed to the transfer portion, during an image formation. Then, the sheet where the toner image has been transferred is conveyed to a fixing portion to fix the image on the sheet. Here, the image forming apparatus has a skew feeding correction portion for correcting skew feeding of the sheet, and displacement of the sheet in a direction (hereinafter referred to as the width direction) perpendicular to the sheet conveying direction, to adjust the posture and the position of the sheet before the sheet is conveyed to the transfer portion.
On the other hand, in recent years, a variety of sheets such as a coated paper, embossed paper, super cardboard and super thin paper has come to be used in an image forming apparatus. Therefore, in an image forming apparatus, it is desired to not only enhance productivity, but also enhance the speed and the precision of skew feeding correction in order to be applicable to all kinds of sheets. Thus in order to enhance the speed and the precision of such skew feeding correction, there has been suggested a skew feeding correction portion of an active skew feeding correction type for correcting the skew feeding during conveying the sheet without stopping the sheet (see U.S. Patent Application Publication No. 2002/0017755 A1).
FIG. 7 illustrates such a conventional configuration of a skew feeding correction portion of an active skew feeding correction type. In this skew feeding correction portion, when activation sensors 27a and 27b and skew feeding detection sensors 28a and 28b detect the tip of a sheet, skew feeding correction motors 23 and 24 start driving in accordance with the detection timing. Accordingly, a pair of skew feeding correction rollers 21 and 22 rolls, and conducts skew feeding correction of a sheet S while it conveys the sheet S.
Then, a pair of registration rollers 30 conducts front edge registration and side edge registration. That is, when the front edge of the sheet S is detected by a registration sensor 131, a registration motor 31 is driven and the roll control of the pair of registration rollers 30 is conducted so as to match the image position with the front edge position of the sheet S on a photosensitive drum (not shown). In addition, the registration shift motor 33 is driven based on a detection signal from a lateral registration detection sensor 35, and the pair of registration rollers 30 is laterally moved so as to match the image position with the front edge position of the sheet S on a photosensitive drum. In this manner, the position of the sheet S is precisely corrected with respect to the image on the photosensitive drum, and subsequently the sheet conveyance is repeatedly conducted.
Incidentally, in an image forming apparatus having such a conventional skew feeding correction portion, it is required to slide the pair of registration rollers 30 in either direction of the width in order to correct the lateral registration of the sheet. Therefore, there is one that has a slide drive portion 55 for sliding the pair of registration rollers 30 using a slide drive belt 57 as shown in FIG. 8A.
Here, the pair of registration rollers 30 is rotatably supported by a bearing 53a provided in a registration slide unit 56, and is configured to receive a rotational driving force through a driving spindle 51 from the slide drive shaft 50 rotated by a registration drive motor M. In addition, the pair of registration rollers 30 is slidable in the width direction along the slide drive shaft 50 through the slide bearing 53.
In addition, the registration slide unit 56 is configured to receive a slide driving force in the width direction through the slide drive belt 57 from the registration slide motor 54, which is a driving source. When the registration slide unit 56 slides in the width direction while the pair of registration rollers 30 nips the sheet S, the pair of registration rollers 30 slides in the width direction along the slide drive shaft 50 while it nips the sheet S.
However, in such a configuration of the slide drive portion 55, once a driving looseness has occurred, the skew feeding correction precision and/or the lateral registration correction precision of the sheet fluctuate. The driving looseness is a looseness possessed by the slide drive portion 55, and it is due to a flexure difference between the tight side and the slack side of the slide drive belt 57, and a fitting looseness (small gap) between the slide shaft 53 and the slide drive shaft 50 shown in FIG. 11, which is described in the following. Once such a driving looseness occurs, a skew feeding of the sheet occurs, resulting in that the lateral registration correction precision and the skew feeding correction precision further fluctuate.
Next, the difference in slide quantity of the sheet when there is a flexure difference at the tight side/slack side of the slide drive belt 57 is described with reference to FIGS. 9A and 9B. FIG. 9A describes a slide quantity of the sheet in a case that the slide directions of the N−1th and Nth sheets S are different, and FIG. 9B describes a slide quantity of the sheet in a case the slide directions of the N−1th and Nth sheets S are the same.
The case that the slide directions are different is a case that, for example, when the slide direction of the N−1th sheet is from the near side to the far side, the slide direction of the next Nth sheet is from the far side to the near side. In addition, the case the slide directions are the same is a case that, for example, when the slide direction of the N−1th sheet is from the near side to the far side, the slide direction of the next Nth sheet is also from the near side to the far side. In the case that the slide directions are the same, the registration slide unit 56 is once moved back to the near side from the far side, and is moved to the far side from the near side to slide the Nth sheet.
When the slide directions of the N−1th and Nth sheets S are different, the relationship between: difference DS between the side edge position of the sheet detected by the sheet position detection means (not shown) and apparatus center 500; and slide quantity D1 of the slide drive portion 55, is DS=D1 as shown in FIG. 9A.
On the other hand, in the case that the slide directions of the N−1th and N sheets are the same, a drive gear 58 for driving a slide drive belt 57 shown in FIG. 8B is rotated in a direction shown in arrow C to once move back from the far side to the near side to slide the Nth sheet. That is, it is rotated in a direction moving back from the slid position to the home position. Subsequently, the drive gear 58 is rotated in a direction opposite to arrow C. Here, upon rotating in this direction, since the drive gear 58 rotates toward the flexure direction of the slide drive belt 57, it cannot transmit a driving force to the registration slide unit 56 for a moment until the belt flexure side shifts to the tight side.
In this case, as shown in FIG. 9B, slide deficiency quantity Δ of the slide drive portion 55 is generated, and the relationship between DS and slide quantity D2 of the slide drive portion 55 becomes DS=D2−Δ. Accordingly, when the directions of the N−1th and Nth sheets are the same direction, slide deficiency quantity Δ is generated.
Here, as slide deficiency quantity Δ is generated, the positioning displacement quantities of the sheets S from the image forming apparatus center become different between the case that the slide directions of the N−1th and Nth sheets are different and the slide directions of the N−1th and Nth sheets are the same. As a result, a difference arises between: left and right margins α1 of a resulting product when the slide directions of the N−1th and Nth sheets are different (shown in FIG. 10A); and left and right margins β1 of a resulting product when the slide directions of the N−1th and Nth sheets are the same (shown in FIG. 10B). Accordingly, a difference arises in the left and right margins between the case that the slide directions of the N−1th and Nth sheets are different and the case that the slide directions of the sheets are the same, resulting in that an image displacement arises.
Next, a description is made of the effect of a fitting looseness between the slide bearing 53 and the slide drive shaft 50. FIG. 11A illustrates the registration slide unit 56 in a state prior to sliding. At this time, the sheet S is being nipped between the pair of registration rollers 30 after skew feeding correction. In this state, when the registration slide unit 56 slides, a fitting looseness between the slide bearing 53 and the drive shaft 50 incurs an inclination in the registration slide unit 56 in the direction of arrow A as shown FIG. 11B, due to the inertia of the sheet S and the registration slide unit itself. Accordingly, a skew feeding with respect to the skew feeding quantity after the skew feeding correction arises by Δ in the sheet S nipped by the pair of registration rollers 30 along with the registration slide unit 56.
At this time, the arisen skew feeding quantity arises in a direction opposite to arrow A as shown in FIGS. 12A and 12B if the slide direction of the registration slide unit 56 is opposite to the case shown in FIGS. 11A and 11B. Accordingly, when the slide correction directions of the sheets are different, skew feedings of skew feeding quantity α2 and skew feeding quantity β2 in different directions arise.
Thus, the invention disclosed herein has been developed in view of such circumstances, and it is intended to provide a sheet conveying apparatus and an image forming apparatus capable of correcting the position of a sheet with high precision.